Radiographic imaging such as x-ray imaging has been used for years in medical applications and for non-destructive testing.
Normally, an x-ray imaging system includes an x-ray source and an x-ray detector system. The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the x-ray detector system. Since some materials absorb a larger fraction of the x-rays than others, an image is formed of the subject or object. The x-ray detector may be of different types, including energy-integrating detectors and photon-counting detectors.
Performance of x-ray imaging detectors are commonly measured using the detective quantum efficiency (DQE). The DQE is defined as the squared signal to noise in the output from the detection system divided by the squared signal-to-noise ratio of the input to the detector, i.e. divided by the squared signal-to-noise ratio that would be measured by an ideal detector. The DQE is a function of spatial frequency in the image. Higher DQE corresponds to better detector performance and less noise in the measured image.
A problem in photon-counting x-ray imaging is that a single photon may cause a pulse to be counted in more than one detector element, sometimes also referred to as a detector pixel or simply pixel, as will be explained later on. This can be caused by several mechanisms. One such mechanism is charge sharing, where the charge cloud generated by a photon interaction is collected by more than one electrode. Another such mechanism is Compton scatter, which causes a photon to deposit energy in a first pixel and then propagate to a second pixel and deposit more energy there. A third mechanism that can cause double counting is fluorescence where an original x-ray photon interaction in a first pixel leaves an inner electron shell of an atom in an excited state, which is subsequently de-excited by the emission of a fluorescence photon, which is reabsorbed in a second pixel. This means that a fraction of the events are counted twice, and since this happens randomly, it degrades the DQE of the detector, thereby giving increased image noise.
Furthermore, double-counting of photons can cause blurring of the image and degrade energy resolution. It is thus an objective to register each photon only once, with the correct photon energy and in the original pixel of interaction.
To achieve this objective, it may be beneficial to implement anti-coincidence logic in the x-ray detector. This anti-coincidence logic can detect simultaneous events and ensure that simultaneous pulses caused by the same photon is only counted once. Such schemes may furthermore be refined so that they use the information contained in the registered set of pulse heights to estimate the original position of interaction and the original photon energy.
A problem with anti-coincidence logic schemes is that they may identify pulses generated by two photons arriving close to each other in time incorrectly, as generated by a single photon. This is called false coincidence, as opposed to true coincidence which is when the anti-coincidence logic correctly identifies two pulses as generated by the same original photon.
False coincidence causes loss of counts and therefore degrades DQE and increases image noise. Furthermore, false coincidence can distort the energy information if the energies of the coincident photons are summed together. If the probability of false coincidence is large enough, the detrimental effect of false coincidence may outweigh the benefits of photon counting.
For a photon-counting detector to be useful in specific applications, such as Computed Tomography (CT), the detector must be able to handle the count rates occurring in the application.
U.S. Pat. No. 6,559,453 relates to a method of enhancing contrast information in x-ray imaging wherein the signals from the photons are given a weight that is influenced by the possibility of charge sharing between adjacent sensor elements.
U.S. Pat. No. 7,214,944 relates to a radiation detection device which compares the temporal overlap of signals from different detector elements, with the objective of making it possible to distinguish real events from false events at high count rates.
U.S. Pat. No. 7,473,902 relate to a method for taking radiographs where charge pulses of bordering pixel units are added together to a total charge pulse.
U.S. Pat. No. 8,050,385 relates to a coincidence detection unit with parameters and thresholds that may have to be adjusted such that the advantage from detecting coincidences is greater than the disadvantage from incorrectly removing false double counts.
U.S. Pat. No. 9,031,197 relates to a method for detecting true coincidence of charge pulses, by allocating the height of the pulses to one of several intervals and analyzing the combination of allocations to intervals in adjacent picture elements.
The publication T. Koenig et al. “Charge Summing in Spectroscopic X-Ray Detectors with High-Z Sensors”, IEEE Transactions on Nuclear Science 60 (6), pp. 4713-4718, 2013, relates to an anti-coincidence logic implementation based on the summation of collected charge in adjacent pixels. This anti-coincidence logic gives improved reconstruction of the incident energy spectrum at low photon fluxes, but causes severe count loss at higher fluxes, above 5·106 counts/mm2·s.
U.S. Pat. No. 9,207,332 relates to an x-ray detector with a low-flux mode where charges collected by neighboring pixels are summed together before being digitized by comparators, and a high-flux mode where no summing of charges from neighboring pixels is made before the signal is digitized by the comparators, but the resulting counts in neighboring pixels are summed together after digitization.
US Patent Application 20160282476A1 relates to an x-ray detector with two counting modes, which initially measures a first count in a first counting mode and, based upon this count value measures a second count value in a second counting mode.
There is however still a need for a detector with improved anti-coincidence logic which gives good image quality both for low and high incident photon flux.